Conventionally, there are communication systems that perform data communication between mobile devices using a GPRS (General Packet Radio Service). Such a communication system is provided with a GGSN (Gateway GPRS Support Node) for connecting different networks, a plurality of SGSNs (Service GPRS Support Nodes) arranged subordinate to the GGSN, and the like, and a mobile device is connected to a network that is an access point via the SGSN and GGSN. Meanwhile, there are communication services in which a communication access fee is paid in advance and communication by a mobile device is enabled within the limit of the paid amount through the use of so-called prepaid mobile devices. Patent Literature 1 describes an example of a prepaid mobile device being applied to a communication system using GPRS.